A) Field
The present invention relates to carbon nanotube, and more particularly to a carbon nanotube containing structure, a method for manufacturing carbon nanotubes, an electric function device, and catalytic fine particles for growing carbon nanotubes.
B) Description of the Related Art
A carbon nanotube is a single-walled or multi-walled cylindrical tube, each wall or layer being graphite carbon atom plane having a thickness of several atom layers, rounded into a tube-shape. The carbon nanotube is a fine structure having an outer diameter of nm order. Carbon nanotubes are applied to various fields because of excellent characteristics such as high electric conductivity, high thermal conductivity and high mechanical strength. Although various studies have been made, the details of growth of carbon nanotubes are still not elucidated.
It is generally considered that metal catalyst functions as nuclei of growth of carbon nanotubes. In addition to pure metal catalyst such as cobalt (Co), iron (Fe) and nickel (Ni), binary metal catalysts such as Co—Mo, Fe—Mo, Fe—Co and Fe—Ni are used in growing carbon nanotubes, particularly single-walled carbon nanotubes (SWNT). Co—Mo and Fe—Mo are combinations of a metal of strong catalytic function, Co and Fe, and Mo having a weak catalytic function. Fe—Co and Fe—Ni are combinations of metals, each having strong catalytic function such as Fe, Co and Ni.
Known growth methods are arc discharge method, laser ablation method, chemical vapor deposition (CVD) method and the like. With the carbon nanotube growing method by CVD, catalytic metal is vapor deposited on a substrate by sputtering or the like and restructured in the form of fine particles by heating, and carbon nanotubes are grown by using catalytic fine particles as nuclei.
JP-A-HEI-9-188509 indicates that secular instability is inevitable in the growth of carbon nanotubes by arc discharge using a carbon rod, and proposes a method for manufacturing carbon nanotubes by which electrodeless discharge plasma is generated, gas having good supply amount controllability such as hydrogen carbide, e.g., methane, is used as carbon source material, and metal catalyst such as iron, nickel and cobalt is supplied separately. The peripheral region of plasm and a substrate for collecting carbon nanotubes are cooled. The metal catalyst is vaporized in plasma, and hydrogen carbide is dissolved. Fine particles are formed in a low temperature area of the plasma peripheral region, and carbon atoms form carbon nanotubes on fine particles by the catalytic function.
JP-A-2005-22886 proposes a carbon nanotube growth method by which on an Si substrate having a via hole etched by using a resist pattern, catalytic fine particles are formed by ablating a target such as Ni, Fe and Co with Nd:YAG laser, annealed and given electric charges, classified by a differential mobility analyzer (DMA) or the like to select catalytic fine particles having a uniform diameter, the catalytic fine particles are blown and deposited on the bottom of the via hole, and the particles on the upper surface are removed together with the resist pattern, to leave carbon nanotubes grown on the catalytic particles deposited in the via hole bottom.
JP-A-2005-285821 proposes a method of forming an underlying layer made of metal of Mo, V, Nb, W or Ti or its metal oxide on a substrate, forming a catalytic layer on the underlying layer, and growing carbon nanotubes on the catalytic layer.
A group of the present inventors has found that growth of multi-walled carbon nanotubes can be promoted greatly by forming a catalytic Co layer on a titanium (Ti) layer.
It has recently been clarified that a Ti layer or an aluminum (Al) layer formed as an underlying layer of catalyst supports the catalytic function and is effective for growth of carbon nanotubes. However, forming the underlying layer such as a Ti layer and an Al layer in a semiconductor device such as an LSI becomes an obstacle in some cases.
The present inventors and colleagues have reported in Chemical Physics Letters, 402 (2005) 149-154 the following thesis. “A Ti—Co alloy target was ablated with Nd:YAG laser and annealed at 1000° C. in a tubular type furnace and in an He atmosphere to form catalytic particles which were classified by a differential mobility analyzer (DMA) to select Ti—Co catalytic fine particles having a uniform diameter. The catalytic fine particles were blown and deposited on a (100) Si substrate, the substrate was heated to a growth temperature (610° C.) in a cooled wall type CVD system, and mixture gas of acetylene and argon (volume ratio: 1:9) was supplied. Carbon nanotubes having the same size as that of the catalytic particles were able to be grown on the catalytic particles.” Diameters of the catalytic fine particles and carbon nanotubes were 5.8 nm (geometric standard deviation: 1.09) and 5.7 nm (geometric standard deviation: 1.13), respectively. The Ti:Co composition was 50:50. It has been found that a carbon nanotube diameter can be controlled by selecting a catalytic particle diameter.
A growth method for carbon nanotubes has not been established clearly yet.